Several methods of liquefying a natural gas stream thereby obtaining LNG are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.
Usually, the natural gas stream to be liquefied (mainly comprising methane) contains ethane, heavier hydrocarbons and possibly other components that are to be removed to a certain extent before the natural gas is liquefied. To this end, the natural gas stream is treated. One of the treatments involves the removal of at least some of the ethane, propane and higher hydrocarbons such as butane and propane.
US 2004/0079107 A1 discloses a process for liquefying natural gas in conjunction with producing a liquid stream containing predominantly hydrocarbons heavier than methane.
A problem of the method disclosed in US 2004/0079107 A1 is that it is rather complicated resulting in relatively high capital expenses (CAPEX). As an example, FIG. 1 of US 2004/0079107 A1 makes use of an intermediate refrigerant cycle 71, thereby relying heavily on external refrigeration. Furthermore the fractionation tower 19 comprises one or more reboilers 20 near the bottom of the tower 19 which heat and vaporize a portion of the liquids flowing down the tower 19 to provide the stripping vapors which flow up the tower 19.